Page 118 - Origin and Prediction of Abnormal Formation Pressures

98 L.A. BURYAKOVSKY, R.D. DJEVANSHIR, G.V. CHILINGAR, H.H. RIEKE III AND J.O. ROBERTSON, JR.

TABLE 4-1
Variation in montmorillonite/kaolinite and montmorillonite/chlorite ratios with depth (data obtained from
N. Batachariya, in Eremenko and Neruchev, 1968)

Depth Montmorillonite/kaolinite ratio Montmorillonite/chlorite ratio
(m)
1200 to 1500 1.0 to 0.4 Chlorites absent
1500 to 1650 0.4 to 0.1 0.7 to 3.3 and higher
>1650 0 0

2
Pressure, kg/cm

I O0 200 300
! ! I
400

800 11

E 1200 ~.4
8
6e ~7 e2
1600
c'-
-4,,,-
~= 2000 %

2400

2800
,C I

3200

Fig. 4-1. Variation of formation pressure with depth in Cambay Basin, India. 1-3 = Calol" 4-8 -- Navagam;
9 = Cambay; 10, 11 - Ankleshvar; 12 = Cosamba. (Modified after Eremenko and Neruchev, 1968, fig. 2,
p. 8.)

According to Rieke and Chilingarian (1974), the AHFP in argillaceous sequences
is often attributed to montmorillonite dehydration as it is altered to hydromica (illite).
For example, according to N. Batacharya (in Eremenko and Neruchev, 1968, p. 8), in
the Cambay Basin of India, where the geothermal gradient reaches 6.5~ m (about
twice as high as those in the Volga-Ural region and West Siberia in Russia: 2.5 and
3.3~ m, respectively) montmorillonite disappears at depths of 1412 to 1500 m
(see Table 4-1). Abnormal formation pressures in the Cambay Basin are presented in